Overcharging

I have a SIM contract with cell c. At the beginning of August they sent me an SMS that im owing them R235.4. I called them on 08/08/2017 with ref no: ********** 492 to enquire and I was told that an agent will call me back, until now my query is not resolved. They claim that there was no money in my acc when they went to debit which is a lie and now they want me to pay for penalties which I do not understand.